1. Field of the Invention
The present invention relates to an electroluminescent display device, and more particularly, to an active matrix organic electroluminescent display device.
2. Discussion of the Related Art
Cathode ray tubes have been commonly used as a display device in televisions and computer monitors. However, the cathode ray tubes are large, heavy, and require a high driving voltage. In contrast, flat panel displays have thin profiles, are light weight, and have low power consumption. The different types of flat panel displays include liquid crystal display (LCD) devices, plasma display panel (PDP) devices, field emission display (FED) devices, and electroluminescence display (ELD) devices.
The ELD devices may be categorized into inorganic electroluminescent display (IELD) devices and organic electroluminescent display (OELD) devices depending on a source material for exciting carriers. The organic electroluminescent displays have developed because of their high brightness, low driving voltage, and production of colors within the visible light range. In addition, the organic electroluminescent displays have a superior contrast ratio because of its self-luminescence. The organic electroluminescent display devices can easily display moving images because of their short microsecond response time and unlimited viewing angle. The organic electroluminescent display devices are stable at low temperatures, and have driving circuitry that can be easily fabricated because of their low voltage driving characteristics. In addition, manufacturing processes of the organic electroluminescent display devices are relatively simple.
In general, OELD emit light by injecting electrons from a cathode electrode and holes from an anode electrode into a luminous layer, combining the electrons with the holes to generate an exciton, and transiting the exciton from an excited state to a ground state. Since the OELD uses a luminous mechanism similar to light emitting diodes, the organic electroluminescence display device may be called an organic light emitting diode (OLED).
The OELD devices may be classified into passive matrix-type and active matrix-type according to a method for driving. The passive matrix-type OLED has a simple structure and is manufactured through a simple process. However, the passive matrix-type OLED devices require high power consumption, thereby limiting overall size. In addition, in the passive matrix-type OELD devices, an aperture ratio decreases according to an increase in a total number of conductive lines. Thus, the passive matrix-type OELD devices are commonly used as small-sized display devices. On the other hand, the active matrix organic electroluminescence display (AMOELD) devices are commonly used in large-sized display devices.
FIG. 1 is an equivalent circuit diagram for a pixel of an active matrix-type organic electroluminescent display (AMOELD) device according to the related art. In FIG. 1, a pixel of an AMOELD device includes a switching thin film transistor (TFT) 5, a driving thin film transistor (TFT) 6, a storage capacitor 7, and an electroluminescent diode 8. A gate electrode of the switching TFT 5 is electrically connected to a gate line 1 and a source electrode of the switching TFT 5 is electrically connected to a data line 2. A drain electrode of the switching TFT 5 is electrically connected to a gate electrode of the driving TFT 6, a drain electrode of the driving TFT 6 is electrically connected to an anode electrode of the electroluminescent diode 8, and a source electrode of the driving TFT 6 is electrically connected to a power line 4. A cathode electrode of the electroluminescent diode 8 is grounded, and the storage capacitor 7 is electrically connected to the gate electrode and the source electrode of the driving TFT 6.
When a signal is applied to the gate electrode of the switching TFT 5 through the gate line 1, the switching TFT 5 turns ON. Accordingly, a signal from the data line 2 is transmitted to the gate electrode of the driving TFT 6 through the switching TFT 5 and is stored in the storage capacitor 7. Then, the driving TFT 6 turns ON by the signal from the data line 2, and a signal from the power line 4 is transmitted to the electroluminescent diode 8 through the driving TFT 6. Therefore, light is emitted from the electroluminescent diode 8. Brightness of the device of FIG. 1 is regulated by controlling current passing through the electroluminescent diode 8. Accordingly, though the switching TFT 5 turns OFF, the driving TFT 6 maintains an ON state due to the signal stored in the storage capacitor 7. Thus, light is emitted by current continuously passing through the electroluminescent diode 8 until the next signal is transmitted to the gate electrode of the driving TFT 6 through the switching TFT 5.
FIG. 2 is a plan view for a pixel of an active matrix-type organic electroluminescent display device according to the related art. In FIG. 2, a gate line 21 and a data line 22 cross each other and define a pixel region “P,” and switching TFT “TS” is formed at the crossing of the gate line 21 and the data line 22, and is connected to the gate line 21 and the data line 22. A driving TFT “TD” is formed within the pixel region “P.” A gate electrode 41 of the driving TFT “TD” is connected to a drain electrode 31 of the switching TFT “TS,” a source electrode 42 of the driving TFT “TD” is connected to a power line 51, and a drain electrode 43 of the driving TFT “TD” is connected to a pixel electrode 61. The power line 51 is formed parallel to the data line 22, and the pixel electrode 61 is formed within the pixel region “P.”
A first capacitor electrode 52 extends from the power line 51 and is disposed within the pixel region “P”. Next, a second capacitor electrode having a first part 71 and a second part 72 is formed, wherein the first and second parts 71 and 72 overlap the power line 51 and the first capacitor electrode 52, respectively, and form a storage capacitor. The second capacitor electrode first part 71 and second part 72 are made of polycrystalline silicon. A partition wall 80 is formed corresponding to the data line 22 and the power line 51 in order to prevent an organic emissive layer (not shown), which will be formed on the pixel electrode 61, from contacting that of the adjacent pixel region “P”.
In the AMOELD device, the capacitance of the storage capacitor should be large to reduce a kick-back voltage that causes poor image display. The capacitance of the storage capacitor is proportional to a size of the electrode of the storage capacitor, and since the power line 51 and the first capacitor electrode 52 being electrodes of the storage capacitor is opaque, an aperture ratio of the AMOELD device is reduced.